Recent observations indicate that the magnetic field of a neutron star decays significantly only when the neutron star is in an interacting binary. This paper reviews the theoretical attempts to model such evolution. Three main possibilities are discussed: the expulsion of the magnetic flux from superconducting core during propeller spin-down in companion's wind, screening of the magnetic field by accreted matter and rapid ohmic decay of crustal magnetic field as a result of heating during accretion. It is found that the screening is unlikely to be effective because of strong Rayleigh-Taylor instabilities. The spindown-induced flux expulsion as well as the crustal heating models remain promising.